The present invention relates to the application of fluid compositions to endless webs, and more particularly to an apparatus and method for applying a fluid coating composition having solids suspended therein, such as a fluid composition containing rupturable microcapsules, to an endless web such as paper. The invention has particular utility in the production of so called carbon-less paper.
Examples of prior art machinery and methods in this field may be found in such references as U.S. Pat. No. 3,186,861, 3,384,536, 3,472,674, 3,630,835, 3,632,378, 3,767,451, 3,897,578, 3,914,511, 4,038,445, and 4,085,237. Reference may be had to these patents for a good general discussion of the difficulties associated with coating microcapsular compositions onto paper webs. Ideally, the least amount of material will be used and will be distributed evenly and uniformly along the surface of the paper. As a practical matter, however, the paper surface is not absolutely smooth. Therefore, mechanical devices for wiping off excess composition as it is being coated may leave too little on the "hills" and an excess in the "valleys". This can be avoided to some extent by using air knives or air jets for metering the coating composition as it is being applied. However, as pointed out, for example, in the '578 reference, air knives also have their disadvantages. The '445 reference discusses the disadvantages of both air knife metering and blade metering.
Another difficulty with applying coatings to paper arises because paper has "depth", in the sense that there are spaces among the various fibers of the paper web. If the micro capsular material penetrates into these spaces, it is no longer available at the web surface, so that excessive coat weights must be used to assure adequate availability of these microcapsular solids at the paper surface. In the '536 reference, penetration of the material is controlled by the chemical formation of a viscous solution of a film-forming polymer. In the '378, '451, and '237 references the web is calendared just prior to application of the material, to smooth and densify the web surface, thereby, helping to overcome some of these disadvantages.
Although a nip can be used, as in the '835 and '511 references, a nip (which is primarily for getting a bite into the paper to drive it) has the disadvantage that the microcapsules must be strong enough to withstand the nip pressures (as mentioned in the '551 reference). Also, the nip pressures tend to drive the microcapsular material into the interstices in the paper web. Air knife coaters require the use of a composition having a relatively low solids content. Thus, fairly large amounts of liquid carrier must be removed from the paper web, in addition to the disadvantages already mentioned.
The more well-known and established coating methods, therefore, all have disadvantages when used for high speed and high-solids content application of capsular formulated coatings. Air-knife coating is both speed and solids limited to levels well below the values otherwise attainable for capsular or formulated coatings. Blade application is primarily limited in the coat weight, requiring higher capsular coat weights than is necessary by other methods to obtain sufficient material uniformity on the surface for uniform surface functional response. Similarly, gravure offset gravure and reverse offset, gravure, all require either higher than necessary capsule coat weights and/or capsule size limitations to achieve the desired product performance. This is also true of reverse roll applications (as exemplified in the '861 and '674 references, both assigned to the assignee of the present invention).
A need therefore remains for a method and apparatus for applying a fluid coating composition having solids suspended therein to an endless web, and which will provide economical and efficient use of the solids while meeting the functional requirements of the coated product, such as uniformity and functional availability (at the surface).
Sheer and/or pressure forces developed during application should be avoided, and a more efficient usage of the solids in the coating composition should be achieved through improved orientation and distribution at the surface of the web material. Preferably, these results should be available for high speed application of coating compositions having a relatively high solids content.